125 Top Core Java Job Interview Questions and Answers

Core Java Interview Questions and Answers:-

1. Define what is the difference between an Inner Class and a Sub-Class?

An Inner class is a class which is nested within another class. An Inner class has access rights for the class which is nesting it and it can access all variables and methods defined in the outer class.
A sub-class is a class which inherits from another class called superclass. Sub-class can access all public and protected methods and fields of its superclass.

2. Define what are the various access specifiers for Java classes?

In Java, access specifiers are the keywords used before a class name which defines the access scope. The types of access specifiers for classes are:

  1. Public: Class, Method, Field is accessible from anywhere.
  2. Protected: Method, Field can be accessed from the same class to which they belong or from the sub-classes, and from the class of the same package, but not from outside.
  3. Default: Method, Field, the class can be accessed only from the same package and not from outside of it’s the native package.
  4. Private: Method, Field can be accessed from the same class to which they belong.

3. Define what’s the purpose of Static methods and static variables?

When there is a requirement to share a method or a variable between multiple objects of a class instead of creating separate copies for each object, we use static keyword to make a method or variable shared for all objects.

4. Define what is data encapsulation and Define what’s its significance?

  • Encapsulation is a concept in Object Oriented Programming for combining properties and methods in a single unit.
  • Encapsulation helps programmers to follow a modular approach for software development as each object has its own set of methods and variables and serves its functions independent of other objects. Encapsulation also serves data hiding purpose.

5. Define what is a singleton class? Give a practical example of its usage.

  • A singleton class in Java can have only one instance and hence all its methods and variables belong to just one instance. Singleton class concept is useful for the situations when there is a need to limit the number of objects for a class.
  • The best example of singleton usage scenario is when there is a limit of having only one connection to a database due to some driver limitations or because of any licensing issues.

6. Define what are Loops in Java? Define what are three types of loops?

Looping is used in programming to execute a statement or a block of statement repeatedly. There are three types of loops in Java:

1) For Loops

For loops are used in java to execute statements repeatedly for a given number of times. For loops are used when a number of times to execute the statements is known to the programmer.

2) While Loops

While loop is used when certain statements need to be executed repeatedly until a condition is fulfilled. In while loops, the condition is checked first before the execution of statements.

3) Do While Loops

Do While Loop is same as While loop with the only difference that condition is checked after execution of a block of statements. Hence in case of doing while loop, statements are executed at least once.

7. Define what is an infinite Loop? Explain what infinite loop is declared?

An infinite loop runs without any condition and runs infinitely. An infinite loop can be broken by defining any breaking logic in the body of the statement blocks.

An infinite loop is declared as follows:

For (;;)


// Statements to execute

// Add any loop breaking logic


8.  Define what is the difference between continuing and break statement?

break and continue are two important keywords used in Loops. When a break keyword is used in a loop, the loop is broken instantly while when continue keyword is used, the current iteration is broken and the loop continues with the next iteration.

In below example, Loop is broken when the counter reaches 4.

For (counter=0;counter

If (counter==4) {



In the below example when counter reaches 4, loop jumps tonext iteration and any statements after the continue keyword are skipped for current iteration.

For (counter=0;counter

If (counter==4) {



System.outprintln(“This will not get printed when counter is 4”);


9. Define what is the difference between double and float variables in Java?

In Java, float takes 4 bytes in memory while Double takes 8 bytes in memory. A float is single precision floating point decimal number while Double is a double precision decimal number.

10.  Define what is Final Keyword in Java? Give an example.

In Java, a constant is declared using the keyword Final. Value can be assigned only once and after assignment, the value of a constant can’t be changed.

In below example, a constant with the name const_val is declared and assigned a value:

Private Final int const_val=100

When a method is declared as final, it can NOT  be overridden by the subclasses. This method is faster than any other method because they are resolved at the compiled time.

When a class is declared as final, it cannot be subclassed. Example String, Integer and other wrapper classes.

11. Define what is the ternary operator? Give an example.

A ternary operator also called a conditional operator is used to decide which value to assign to a variable based on a Boolean value evaluation. It’s denoted as?

In the below example, if rank is 1, status is assigned a value of “Done” else “Pending”.

public class conditionTest {
public static void main(string args[]) {
String status;
int rank;
status= (rank == 1) ? “Done”: “Pending”;

12: Define what are 6 different types of operators in Java?

In Java, operators can be classified in the following six types:

Arithmetic Operators
Used for arithmetic calculations. Example are +,-,*,/,%,++,–

Relational Operators
Used for relational comparison. E.g. ==,!=, >,<,<=,>=

Bitwise operators
Used for a bit by bit operations. E.g. &,|,^,~

Logical Operators
Used for logical comparisons. E.g. &&,||,!

Assignment Operators
Used for assigning values to variables. E.g. =,+=,-=,*=,/=

13. Define what is default switch case? Give an example.

In a switch statement, the default case is executed when no other switch condition matches. The default case is an optional case.
It can be declared only once all other switch cases have been coded.

In the below example, when score is not 1 or 2, default case is used.

public class switchExample {
int score=4;
public static void main(String args[]) {
switch (score) {
case 1:
System.out.println(“Score is 1”);
case 2:
system.out.println(“Score is 2”);
System.out.println(“Default Case”);



14.  Define what’s the base class in Java from which all classes are derived?


15. Can main() method in Java can return any data?

In Java, the main() method can’t return any data and hence, it’s always declared with a void return type.

16. Define what are Java Packages? Define what’s the significance of packages?

In Java, a package is a collection of classes and interfaces which are bundled together as they are related to each other. Use of packages helps developers to modularize the code and group the code for proper re-use. Once the code has been packaged in Packages, it can be imported in other classes and used.

17.  Can we declare a class as Abstract without having any abstract method?

Yes, we can create an abstract class by using an abstract keyword before class name even if it doesn’t have any abstract method. Explain whatever, if a class has even one abstract method, it must be declared as abstract otherwise it will give an error.

18. Define what’s the difference between an Abstract Class and Interface in Java?

  • The primary difference between an abstract class and interface is that an interface can only possess declaration of public static methods with no concrete implementation while an abstract class can have members with any access specifiers (public, private, etc) with or without a concrete implementation.
  • Another key difference in the use of abstract classes and interfaces is that a class which implements an interface must implement all the methods of the interface while a class which inherits from an abstract class doesn’t require the implementation of all the methods of its superclass.
  • A class can implement multiple interfaces but it can extend only one abstract class.

19. Define what are the performance implications of Interfaces over abstract classes?

  • Interfaces are slower in performance as compared to abstract classes as extra indirections are required for interfaces. Another key factor for developers to take into consideration is that any class can extend only one abstract class while a class can implement many interfaces.
  • Use of interfaces also puts an extra burden on the developers as any time an interface is implemented in a class; the developer is forced to implement each and every method of the interface.

20. Does Importing a package imports its sub-packages as well in Java?

  • In Java, when a package is imported, its sub-packages aren’t imported and developer needs to import them separately if required.
  • For example, if a developer imports a package university.*, all classes in the package named university are loaded but no classes from the sub-package are loaded. To load the classes from its sub-package ( say department), the developer has to import it explicitly as follows:

Import university.department.*

21. Can we declare the main method of our class as private?

In Java, the main method must be public static in order to run any application correctly. If the main method is declared as private, the developer won’t get any compilation error Explain whatever, it will not get executed and will give a runtime error.

22.  Explain what can we pass the argument to a function by reference instead of pass by value?

In Java, we can pass the argument to a function only by value and not by reference.

23. Explain what an object is serialized in java?

In Java, to convert an object into byte stream by serialization, an interface with the name Serializable is implemented by the class. All objects of a class implementing serializable interface get serialized and their state is saved in the byte stream.

24. When we should use serialization?

Serialization is used when data needs to be transmitted over the network. Using serialization, object’s state is saved and converted into byte stream.T he byte stream is transferred over the network and the object is re-created at the destination.

25. Is it compulsory for a Try Block to be followed by a Catch Block in Java for Exception handling?

Try block needs to be followed by either Catch block or Finally block or both. Any exception thrown from try block needs to be either caught in the catch block or else any specific tasks to be performed before code abortion are put in the Finally block.

26. Is there any way to skip Finally block of an exception even if some exception occurs in the exception block?

If an exception is raised in a Try block, control passes to catch block if it exists otherwise to finally block. Finally block is always executed when an exception occurs and the only way to avoid execution of any statements in Finally block is by aborting the code forcibly by writing following line of code at the end of try block:


27. When the constructor of a class is invoked?

The constructor of a class is invoked every time an object is created with a new keyword.

For example, in the following class two objects are created using new keyword and hence, constructor is invoked two times.

public class const_example {

const_example() {

System.out.println(“Inside constructor”);


Public static void main(String args[]) {

const_example c1=new const_example();

const_example c2=new const_example();



28. Can a class have multiple constructors?

Yes, a class can have multiple constructors with different parameters. Which constructor gets used for object creation depends on the arguments passed while creating the objects.

29. Can we override static methods of a class?

We cannot override static methods. Static methods belong to a class and not to individual objects and are resolved at the time of compilation (not at runtime).Even if we try to override static method,we will not get an complitaion error,nor the impact of overriding when running the code.

30. In the below example, Define what will be the output?

public class superclass {

public void displayResult() {

System.out.println(“Printing from superclass”);



public class subclass extends superclass {

public void displayResult() {

System.out.println(“Displaying from subClass”);



public static void main(String args[]) {

subclass obj=new subclass();




Output will be:

Displaying from subclass

Displaying from superclass

31. Is String a data type in java?

The string is not a primitive data type in java. When a string is created in java, it’s actually an object of Java.Lang.String class that gets created. After the creation of this string object, all built-in methods of String class can be used on the string object.

32.  In the below example, Explain what many String Objects are created?

String s1=” I am Java Expert”;

String s2=” I am C Expert”;

String s3=” I am Java Expert”;

In the above example, two objects of Java.Lang.String class is created. s1 and s3 are references to the same object.

33. Why Strings in Java are called as Immutable?

In Java, string objects are called immutable as once the value has been assigned to a string, it can’t be changed and if changed, a new object is created.

In below example, reference str refers to a string object having a value “Value one”.

String str=” Value One”;

When a new value is assigned to it, a new String object gets created and the reference is moved to the new object.

str=” New Value”;

34. Define what’s the difference between an array and a Vector?

An array groups data of same primitive type and is static in nature while vectors are dynamic in nature and can hold data of different data types.

35. Define what is multithreading?

Multithreading is a programming concept to run multiple tasks in a concurrent manner within a single program. Threads share the same process stack and running in parallel. It helps in performance improvement of any program.

36. Why Runnable Interface is used in Java?

A runnable interface is used in Java for implementing multi-threaded applications. Java.Lang. The Runnable interface is implemented by a class to support multithreading.

37. Define what are the two ways of implementing multi-threading in Java?

Multi-threaded applications can be developed in Java by using any of the following two methodologies:

1. By using Java.Lang.Runnable Interface. Classes implement this interface to enable multithreading. There is a Run() method in this interface which is implemented.

2. By writing a class that extends Java.Lang.Thread class.

38. When a lot of changes are required in data, which one should be a preference to be used? String or StringBuffer?

Since StringBuffers are dynamic in nature and we can change the values of StringBuffer objects unlike String which is immutable, it’s always a good choice to use StringBuffer when data is being changed too much. If we use String in such a case, for every data change a new String object will be created which will be extra overhead.

39. Define what’s the purpose of using Break in each case of Switch Statement?

A break is used after each case (except the last one) in a switch so that code breaks after the valid case and doesn’t flow in the proceeding cases too.

If the break isn’t used after each case, all cases after the valid case also get executed resulting in wrong results.

40.  Explain what garbage collection is done in Java?

In Java, when an object is not referenced anymore, garbage collection takes place and the object is destroyed automatically. For automatic garbage collection java calls either System.gc() method or Runtime.gc() method.

41. Explain what we can execute any code even before the main method?

If we want to execute any statements before even the creation of objects at load time of class, we can use a static block of code in the class. Any statements inside this static block of code will get executed once at the time of loading the class even before the creation of objects in the main method.

42. Can a class be a superclass and a sub-class at the same time? Give an example.

If there is a hierarchy of inheritance used, a class can be a superclass for another class and a sub-class for another one at the same time.

In the example below, a continent class is sub-class of world class and it’s a superclass of country class.

public class world {



public class continent extends world {



the public class country extends continent {



43.  Explain what objects of a class are created if no constructor is defined in the class?

Even if no explicit constructor is defined in a java class, objects get created successfully as a default constructor is implicitly used for object creation. This constructor has no parameters.

44.  In multi-threading Explain what can we ensure that a resource isn’t used by multiple threads simultaneously?

In multi-threading, access to the resources which are shared among multiple threads can be controlled by using the concept of synchronization. Using the synchronized keyword, we can ensure that only one thread can use a shared resource at a time and others can get control of the resource only once it has become free from the other one using it.

45. Can we call the constructor of a class more than once for an object?

A constructor is called automatically when we create an object using the new keyword. It’s called only once for an object at the time of object creation and hence, we can’t invoke the constructor again for an object after its creation.

46. There are two classes named classA and classB. Both classes are in the same package. Can a private member of the class be accessed by an object of class B?

Private members of a class aren’t accessible outside the scope of that class and any other class even in the same package can’t access them.

47. Can we have two methods in a class with the same name?

We can define two methods in a class with the same name but with different number/type of parameters. Which method is to get invoked will depend upon the parameters passed.

For example in the class below we have two print methods with same name but different parameters. Depending upon the parameters, appropriate one will be called:

public class methodExample {

public void print() {

System.out.println(“Print method without parameters.”);


public void print(String name) {

System.out.println(“Print method with paramter”);


public static void main(String args[]) {

methodExample obj1=new methodExample();





48. Explain what can we make copy of a java object?

We can use the concept of cloning to create a copy of an object. Using clone, we create copies with the actual state of an object.

Clone() is a method of Cloneable interface and hence, Cloneable interface needs to be implemented for making object copies.

49. Define what’s the benefit of using inheritance?

A key benefit of using inheritance is reusability of code as inheritance enables sub-classes to reuse the code of its superclass. Polymorphism (Extensibility ) is another great benefit which allows new functionality to be introduced without affecting existing derived classes.

50.  Define what’s the default access specifier for variables and methods of a class?

Default access specifier for variables and method is package protected i.e variables and class is available to any other class but in the same package, not outside the package.

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52.  Explain what can we restrict inheritance for a class so that no class can be inherited from it?

If we want a class not to be extended further by any class, we can use the keyword Final with the class name.

In the following example, Stone class is Final and can’t be extend

</em>public Final Class Stone {

// Class methods and Variables


54. Define what’s difference between Stack and Queue?

Stack and Queue both are used as the placeholder for a collection of data. The primary difference between a stack and a queue is that stack is based on the Last in First out (LIFO) principle while a queue is based on FIFO (First In First Out) principle.

55. In Java, Explain what we can disallow serialization of variables?

If we want certain variables of a class not to be serialized, we can use the keyword transient while declaring them. For example, the variable trans_var below is a transient variable and can’t be serialized:

public class transientExample {

private transient trans_var;

// rest of the code


56.  Explain what can we use primitive data types as objects?

Primitive data types like it can be handled as objects by the use of their respective wrapper classes. For example, Integer is a wrapper class for primitive data type int. We can apply different methods to a wrapper class, just like any other object.

57. Which types of exceptions are caught at compile time?

Checked exceptions can be caught at the time of program compilation. Checked exceptions must be handled by using a try-catch block in the code in order to successfully compile the code.

58. Describe different states of a thread.

A thread in Java can be in either of the following states:

  • Ready: When a thread is created, it’s in Ready state.
  • Running: A thread currently being executed is in running state.
  • Waiting: A thread waiting for another thread to free certain resources is in waiting for the state.
  • Dead: A thread which has gone dead after execution is in the dead state.

59. Can we use a default constructor of a class even if an explicit constructor is defined?

Java provides a default no argument constructor if no explicit constructor is defined in a Java class. But if an explicit constructor has been defined, default constructor can’t be invoked and a developer can use only those constructors which are defined in the class.

60. Can we override a method by using the same method name and arguments but different return types?

The basic condition of method overriding is that method name, arguments, as well as a return type, must be exactly the same as is that of the method being overridden.  Hence using a different return type doesn’t override a method.

61. Define what will be the output of the following piece of code?

public class operator example {

public static void main(String args[]) {

int x=4;




In this case, postfix ++ operator is used which first returns the value and then increments. Hence it’s output will be 4.

61. Does a person say that he compiled a java class successfully without even having the main method in it? Is it possible?

the main method is an entry point of the Java class and is required for the execution of the program Explain whatever; a class gets compiled successfully even if it doesn’t have a main method. It can’t be run through.

62.  Can we call a non-static method from inside a static method?

Non-Static methods are owned by objects of a class and have object level scope and in order to call the non-static methods from a static block (like from a static main method), an object of the class needs to be created first. Then using object reference, these methods can be invoked.

63. Define what are the two environment variables that must be set in order to run any Java programs?

Java programs can be executed in a machine only once following two environment variables have been properly set:

  1. PATH variable
  2. CLASSPATH variable

64. Can variables be used in Java without initialization?

In Java, if a variable is used in code without prior initialization by a valid value, the program doesn’t compile and gives an error as no default value is assigned to variables in Java.

65. Can a class in Java be inherited from more than one class?

In Java, a class can be derived from only one class and not from multiple classes. Multiple inheritances are not supported by Java.

66. Can a constructor have a different name than a Class name in Java?

The constructor in Java must have the same name as the class name and if the name is different, it doesn’t act as a constructor and compiler think of it as a normal method.

67. Define what will be the output of Round(3.7) and Ceil(3.7)?

Round(3.7) returns 3 while Ceil(3.7) returns 4.

68: Can we use goto in Java to go to a particular line?

In Java, there is not goto keyword and java doesn’t support this feature of going to a particular labeled line.

69. Can a dead thread be started again?

In Java, a thread which is in the dead state can’t be started again. There is no way to restart a dead thread.

70. Is the following class declaration correct?

public abstract final class testClass {

// Class methods and variables


The above class declaration is incorrect as an abstract class can’t be declared as Final.

71. Is JDK required on each machine to run a Java program?

JDK is development Kit of Java and is required for development only and to run a Java program on a machine, JDK isn’t required. Only JRE is required.

72. Define what’s the difference between comparison done by equals method and == operator?

In Java, equals() method is used to compare the contents of two string objects and returns true if the two have the same value while == operator compares the references of two string objects.

In the following example, equals() returns true as the two string objects have same values. Explain whatever == operator returns false as both string objects are referencing to different objects:

public class equalsTest {

public static void main(String args[]) {

String srt1=”Hello World”;

String str2=”Hello World”;

If (str1.equals(str2))

{// this condition is true

System.out.println(“str1 and str2 are equal in terms of values”);


If (str1==str2) {

//This condition is not true

System.out.println(“Both strings are referencing same object”);




// This condition is true

System.out.println(“Both strings are referencing different objects”);



73. Is it possible to define a method in Java class but provide it’s implementation in the code of another language like C?

Yes, we can do this by use of native methods. In the case of native method based development, we define public static methods in our Java class without its implementation and then the implementation is done in another language like C separately.

74.  Explain what destructors are defined in Java?

In Java, there are no destructors defined in the class as there is no need to do so. Java has its own garbage collection mechanism which does the job automatically by destroying the objects when no longer referenced.

75. Can a variable be local and static at the same time?

No, a variable can’t be static as well as local at the same time. Defining a local variable as static gives a compilation error.

76. Can we have static methods in an Interface?

Static methods can’t be overridden in any class while any methods in an interface are by default abstract and are supposed to be implemented in the classes being implementing the interface. So it makes no sense to have static methods in an interface in Java.

77. In a class implementing an interface, can we change the value of any variable defined in the interface?

No, we can’t change the value of any variable of an interface in the implementing class as all variables defined in the interface are by default public, static and Final and final variables are like constants which can’t be changed later.

78. Is it correct to say that due to the garbage collection feature in Java, a java program never goes out of memory?

Even though automatic garbage collection is provided by Java, it doesn’t ensure that a Java program will not go out of memory as there is a possibility that creation of Java objects is being done at a faster pace compared to garbage collection resulting in filling of all the available memory resources.

So, garbage collection helps in reducing the chances of a program going out of memory but it doesn’t ensure that.

79. Can we have any other return type than void for the main method?

No, Java class main method can have only void return type for the program to get successfully executed.

Nonetheless, if you absolutely must return a value to at the completion of the main method, you can use System.exit(int status)

80. I want to re-reach and use an object once it has been garbage collected. Explain what it’s possible?

Once an object has been destroyed by the garbage collector, it no longer exists on the heap and it can’t be accessed again. There is no way to reference it again.

81. In Java thread programming, which method is a must implementation for all threads?

Run() is a method of Runnable interface that must be implemented by all threads.

82. I want to control database connections in my program and want that only one thread should be able to make database connection at a time. Explain what can I implement this logic?

This can be implemented by use of the concept of synchronization. Database related code can be placed in a method which is synchronized keyword so that only one thread can access it at a time.

83. Explain what can an exception be thrown manually by a programmer?

In order to throw an exception in a block of code manually, throw keyword is used. Then this exception is caught and handled in the catch block.

public void topMethod(){
}catch(ManualException e){ }

public void excMethod{
String name=null;
if(name == null){
throw (new ManualException(“Exception thrown manually “);

84.  I want my class to be developed in such a way that no other class (even derived class) can create its objects. Explain what can I do so?

If we declare the constructor of a class as private, it will not be accessible by any other class and hence, no other class will be able to instantiate it and formation of its object will be limited to itself only.

85. Explain what objects are stored in Java?

In Java, each object when created gets memory space from a heap. When an object is destroyed by a garbage collector, the space allocated to it from the heap is re-allocated to the heap and becomes available for any new objects.

86. Explain what can we find the actual size of an object on the heap?

In Java, there is no way to find out the exact size of an object on the heap.

87. Which of the following classes will have more memory allocated?

Class A: Three methods, four variables, no object

Class B: Five methods, three variables, no object

Memory isn’t allocated before the creation of objects. Since for both classes, there are no objects created so no memory is allocated on the heap for any class.

88. Define what happens if an exception is not handled in a program?

If an exception is not handled in a program using try-catch blocks, the program gets aborted and no statement executes after the statement which caused exception throwing.

89.  I have multiple constructors defined in a class. Is it possible to call a constructor from another constructor’s body?

If a class has multiple constructors, it’s possible to call one constructor from the body of another one using this().

90. Define what’s meant by an anonymous class?

An anonymous class is a class defined without any name in a single line of code using the new keyword.

For example, in below code we have defined an anonymous class in one line of code:

public java.util.Enumeration testMethod()


return new java.util.Enumeration()



public boolean hasMoreElements()


// TODO Auto-generated method stub

return false;



public Object nextElement()


// TODO Auto-generated method stub

return null;



91. Is there a way to increase the size of an array after its declaration?

Arrays are static and once we have specified its size, we can’t change it. If we want to use such collections where we may require a change of size ( no of items), we should prefer vector over the array.

92. If an application has multiple classes in it, is it okay to have a main method in more than one class?

If there is the main method in more than one classes in a java application, it won’t cause any issue as the entry point for any application will be a specific class and code will start from the main method of that particular class only.

93. I want to persist data of objects for later use. Define what’s the best approach to do so?

The best way to persist data for future use is to use the concept of serialization.

94. Define what is a Local class in Java?

In Java, if we define a new class inside a particular block, it’s called a local class. Such a class has local scope and isn’t usable outside the block where it’s defined.

95. String and StringBuffer both represent String objects. Can we compare String and StringBuffer in Java?

Although String and StringBuffer both represent String objects, we can’t compare them with each other and if we try to compare them, we get an error.

96. Which API is provided by Java for operations on a set of objects?

Java provides a Collection API which provides many useful methods which can be applied on a set of objects. Some of the important classes provided by Collection API include ArrayList, HashMap, TreeSet, and TreeMap.

97. Can we cast any other type to Boolean Type with typecasting?

No, we can neither cast any other primitive type to Boolean data type nor can cast Boolean data type to any other primitive data type.

98.  Can we use different return types for methods when overridden?

The basic requirement of method overriding in Java is that the overridden method should have the same name and parameters. But a method can be overridden with a different return type as long as the new return type extends the original.

For example, the method is returning a reference type.

Class B extends A{

A method(int x){

//original method


B method(int x){

//overridden method



99. Define what’s the base class of all exception classes?

In Java, Java.Lang.throwable is the superclass of all exception classes and all exception classes are derived from this base class.

100. Define what’s the order of call of constructors in inheritance?

In case of inheritance, when a new object of a derived class is created, first the constructor of the superclass is invoked and then the constructor of the derived class is invoked.